The present invention relates to an optical memory system for recording, reading or erasing information by irradiating an optical disc such as magneto-optical disc, with light emitted from a semiconductor laser, and more particularly, to a beam control device of a semiconductor laser used in an optical memory system and the control method thereof.
Recently optical discs, magneto-optical discs and many others have been developed as systems making use of laser beam.
For example, a magneto-optical disc memory device is an optical recording and reading apparatus for recording, reading or erasing by using a magnetic film having an axis of easy magnetization perpendicular to the surface of the magnetic film as the recording medium, and irradiating it with a laser beam. The recording and reading methods of a magneto-optical disc memory device are explained below.
In recording, the information is recorded as follows. A laser beam focused to a spot of a diameter of about 1 .mu.m is modulated in intensity depending on the information signal and is emitted to the magnetic film surface to elevate the temperature of the magnetic film locally. In this heated area, the coercive force decreases, and therefore, when an auxiliary magnetic field is applied simultaneously from an outside source, the direction of a magnetization is inverted, and the information is recorded.
In reading, on the other hand, the recorded magnetic film surface is irradiated with al linearly polarized light of laser beam having a smaller quantity of light than when recording. As a result, the reflected light produces an inclination of polarization due to the magneto-optical effect of magnetic film (Kerr effect). This inclination is passed through an analyzer to be converted into a change in intensity of light. When the change is detected by a photo detector, a read out signal is obtained.
The light intensity of laser beam requires two output levels, that is, high and low, as discussed above. Namely, the output light intensity level is high when recording (erasing), and is low when reading.
The semiconductor laser emitting a laser beam possesses a temperature characteristic, that varies intensity of laser beam depending on the ambient temperature. In other words, when the ambient temperature changes, the ratio of polarization (the light intensity of P-polarized component/the light intensity of S-polarized component) fluctuates. When the high output light intensity of laser beam varies in the information recording mode, insufficient or excessive writing of information on the recording medium may occur, thereby deteriorating the reliability of the read out information. On the other hand, when the low output light intensity varies in information reading mode, the S/N ratio of the read out signal deteriorates. Therefore, it is necessary to stabilize the light intensity of laser beam at both high and low output levels.
Therefore, conventionally, in this sort of system, the output light from the laser light source is detected by a monitor detector, and the driving current of the laser light source is controlled depending on this result of detection so as to stabilize the laser beam output.
For example, generally, the laser beam is controlled in the following two manners.
(1) The method of recording the laser beam oscillated from the back side of a resonator of semiconductor laser by a photo detector for monitoring and controlling the output light intensity at the front side of the resonator on the basis of the light intensity information obtained from this photo detector.
(2) The method of recording part of the output light at the front side of a resonator of semiconductor laser by using a beam splitter or the like by a detector for monitoring and controlling the output light intensity at the front side of the resonator on the basis of the light intensity information obtained from this detector.
Anyway, in the systems making use of laser beam, it is general to extracting only a specific polarized component in a laser beam. For example, in a magneto-optical system, the information is recorded or read out by using the P-polarized component of the laser beam.
In such conventional laser beam controlling methods, however, since the light entering the photo detector for monitoring contains other polarized components aside from a specific polarized component, the light intensity information obtained in the photo detector for monitoring includes light intensity information obtained on the basis of other polarized components other than the actually required specific polarized component.
Accordingly, in an optical system for recording, reading and/or erasing the information by a laser beam of a specific polarized component, when the laser beam is controlled by light intensity information including light intensity information other than the specific polarized component, the laser beam of the specific polarized component delivered from the laser is influenced by the light intensity of other polarized components, so that the laser beam may not be controlled accurately. That is, the light intensity of laser beam necessary for recording, reading and/or erasing cannot be applied to the optical disc.
In a system using a laser beam of specific polarized component, a more accurate control of the output of laser beam may be realized by obtaining the light intensity information acquired by utilizing a photo detector for monitoring the specific polarized component being used (the P-polarized component in a magneto-optical system) and controlling according to this light intensity information. It is possible to control accurately the specific polarized component in the laser beam outputted from the laser to the optical disc.
Below, in the magneto-optical system in which a specific polarized component is a P-polarized component, possible problems are discussed.
In the conventional method, the light entering the photo detector for monitoring contains both P-polarized component and S-polarized component. Therefore, if the ratio of the P-polarized component to the S-polarized component in the laser beam the polarization ratio) is always constant, the laser beam output light intensity of the P-polarized component can be controlled by the methods (1), (2) above. Actually, however, since the polarization ratio increases or decreases depending on the changes of the ambient temperature of the laser, the conventional light intensity information from the photo detector for monitoring comprising the P-polarized component and S-polarized component is different from the light intensity of the laser beam of the actually used P-polarized component (the light intensity of laser beam outputted to the magneto-optical disc). This phenomenon occurs not only due to variations of the polarization ratio, but also due to rotation of the direction of polarization of the laser output beam.
FIG. 4 shows the relationship between laser driving current (I) and laser output intensity (power) at various ambient temperatures (Tc) of the semiconductor laser. As understood from this diagram, when the temperature rises, the driving current must be increased in order to obtain a specified laser output intensity (for example, about 3 mW when reading, or about 30 mW when recording or erasing).
If the laser is driven at a driving current I' (mA) and the ambient temperature of the laser (T.degree. C.) is not changed, the laser output comprising both P-polarized component and S-polarized component becomes a specified level P1. If the laser is driven at the same driving current (I') and the laser ambient temperature (T) rises, the laser output is lower than level P1, or if the laser ambient temperature (T) drops, the laser output is higher than level P1 (FIG. 5 (1)). Meanwhile, the polarization ratio remains constant if the laser is driven by the driving current I' (mA) and the laser ambient temperature (T.degree. C.) does not change. At this time, as for the P-polarized component, the light intensity of laser beam is a specified level P2. If the laser is driven by the same driving current and the laser ambient temperature rises, the light intensity of laser beam of P-polarized component is lower than level P2, and if the laser ambient temperature drops, the light intensity of laser beam of P-polarized component is higher than level P2 (FIG. 5 (2)).
Thus, according to the conventional stabilizing method of light intensity of laser beam, that is, when the light intensity of laser beam comprising both P-polarized component and S-polarized component are detected by a photo detector for monitoring, and the laser driving current is controlled according to this result of detection, the light intensity of laser output comprising the P-polarized component and S-polarized component can be maintained at a specified level of P1 at each temperature as shown in FIG. 6 (1). But the light intensity of P-polarized component is influenced by the S-polarized component because the light intensity of laser beam is controlled by detecting both P-polarized component and S-polarized component, and it cannot be kept at a specified level of P2 as shown in FIG. 6 (2), that is, the light intensity of laser beam is higher than level P2 when the laser ambient temperature is lower, and is lower than level P2 when the laser ambient temperature is higher. Therefore, in spite of correction of the light intensity of laser beam, the light intensity of the P-polarized component in the laser beam given from the laser to the magneto-optical disc cannot be adjusted to the specified level P2, so that the temperature variations may adversely affect reading recording or erasing as stated above. Incidentally, the graphs in FIGS. 5 (1), (2) and FIGS. 6(1), (2) schematically show the increase and decrease due to temperature changes.
Thus, conventionally, since the light intensity information from the photo detector for monitoring is different from the actually used light intensity, it is impossible to control the light intensity of laser beam accurately by the control on the basis of the above light intensity information.